1. Field of the Invention
The present invention relates in general to electronic devices, and more specifically to a voltage controller oscillator circuit with low noise gain control.
2. Description of the Related Art
Many different frequency bands are known and in use today for mobile wireless applications, including the Universal Mobile Telecommunications System (UMTS) band operating in the 1.920–2.170 gigahertz (GHz) frequency range, the Personal Communications Services (PCS) band operating in the 1.850–1.990 GHz frequency range, the Digital Communications Services (DCS) band operating in the 1.710–1.880 GHz frequency range, the “900 band” operating in the 880–960 megahertz (MHz) frequency range, the “800 band” operating in the 824–894 MHz frequency range, etc., where each listed frequency band includes both transmit and receive frequencies. Mobile phone manufactures often need to design phones that are operable in multiple frequency bands for use by one or more different mobile phone service providers. Chip manufacturers desire to design and build chip sets that may be used for any phone and/or for any service provider anywhere in the world. Similar needs exist for other wireless and wireline applications.
The heart of a wireless chip is the phase locked loop (PLL), and an essential component of the PLL is the voltage controlled oscillator (VCO). The gain factor “KV” of a conventional multi-band VCO changes commensurate with frequency changes as the VCO is switched from one frequency band to another. It has been very difficult to keep the VCO gain within a specified range when band switching over a large frequency range. The problem of maintaining a relatively constant gain is compounded by the fact that a single discrete PLL filter is typically used for all bands on the same printed circuit board (PCB). PLL dynamics dictate that the VCO gain should be as constant as possible over the entire operable frequency range. The conventional VCO was designed for a fixed supply voltage. A current source has often been used for VCO bias and sometimes as a shut-off mechanism for the VCO. It has been determined, however, that a very large percentage (e.g., approximately 70%) of the noise in conventional VCOs is attributable to this current source.
It is desired to keep the gain factor KV as constant as possible over the entire operable frequency range of the VCO. It is desired to be able to perform KV calibration at any time during operation and over operating temperature ranges. It is desired to be able to control the KV variation to achieve improved PLL performance and to optimize current drain in the VCO. It is desired to provide a VCO for a wide range of applications, including modern wireless transceivers which tend to require wide frequency range coverage. And it is desired to enable the design of fully integrated, multi-band direct-conversion transmitters for existing and future wireless and wireline applications.